1. Field of the Invention
The preferred embodiments of the present invention generally relate to communications and electronics cabling, and in particular to a vapor proof cable, such as for high speed communications and network interconnect cable, and a method of manufacturing the same.
2. Background Art
Communications and electronics cables are used today in a broad array of applications, many of which require that the cable carry high frequency signals over long distances. The operating frequency range for modem cable is significantly higher than the range needed for past applications, due in part to the evolution of communications and electronics equipment. In addition, today""s applications require that cable operate under environmental conditions that are significantly more demanding than in the past.
Communications and electronics applications have been proposed that require cables capable of supporting ethernet protocols, while submerged for extended periods of time in fluid, such as oil, gas, water and the like. In at least one application, networking cables are installed at gasoline service stations to interconnect fuel pump electronics and point of sale (POS) equipment. The point of sale equipment communicates with the fuel pump via an ethernet data transmission protocol, such as established in accordance with the IEEE 802.3 10Base-T standard. Interconnect cable used in service station applications is exposed to petroleum fumes and, in some instances, may be submerged in fuel. Other protocols that cable can be used for include asynchronous transfer mode communication.
Heretofore, local area networks, such as used at service stations, typically use category 5 cable as the interconnect cable. Category 5 represents a standard set forth by ANSI, and the TIA/EIA group. Conventional category 5 cable includes twisted groups of insulated conductors. Each twisted group may include two or more conductors forming pairs. Twisted pair cable includes air gaps between an inner surface of the cable jacket and the twisted pair insulated conductors. Twisted pair cable also includes a hollow core between the multiple twisted pair insulated conductors within the cable. The air gaps and hollow core both facilitate the migration of fumes or vapors along the length of the cable. Hence, the potential exists that the cable may transport explosive vapors from the pump to the facility where the clerk is located.
In the past, attempts have been made to vapor proof category 5 cable in order to prevent fumes from migrating to the service station and to comply with safety regulations. One method in the past includes stripping away the cable jacket at multiple discrete regions along the length of the cable when the cable is installed to expose the insulated conductors. A potting material is applied to the conductors at each exposed area to form a vapor blocking seal. The potting material is applied at multiple discrete points along the length of the cable to provide a series of discrete or sectional vapor locks. Multiple vapor locks are necessary since the potting material may develop cracks or be improperly applied, thereby permitting vapor to enter the cable and migrate through a vapor lock. Also, the jacket may become damaged between the service station and any given vapor lock, thereby permitting vapor to enter the jacket and migrate toward the service station upstream of a vapor lock. The existing practice of stripping cables and adding potting material is labor intensive, expensive and unreliable and is undesirable.
FIG. 1 illustrates a category 5 cable that has been used for ATM and ethernet interconnections heretofore. The cable 10 includes a jacket 12 enclosing four twisted pairs 14-17 of conductors arranged in a helix configuration and surrounding a hollow core 18. The twisted pairs 14-17 contact one another and the inner surface 20 of the jacket 12. The relative positions of the twisted pairs 14-17 remain substantially constant with respect to one another. The twisted pairs 14-17 are also twisted to form one large helix. The outer boundary of each twisted pair 14-17 is denoted by dashed line 28. Do to the very nature of a helix, the cable 10 includes several peripheral air gaps 24-27 located between the inner surface 20 of the jacket 10 and the outer peripheral sections of the twisted pairs 14-17, and air gaps 38 within each twisted pair 14-17.
Each twisted pair 14-17 comprises two wires 30 and 32 enclosed in insulators 34 and 36, respectively. A rip cord (not shown) may be provided proximate the inner surface 20 of the jacket 12. The wires 30 and 32 are copper and the insulators 34 and 36 are formed of a polyolefin or fluoropolymer insulator. The jacket 12 is constructed of riser or plenum rated PVC or fluoropolymer.
The cable 10 is arranged in a specific geometry and constructed from materials having a set of desired electrical and physical properties that interact with one another in a particular manner. The overall geometric and material combination affords physical and electrical characteristics that satisfy the requirements of the category 5 standard. Therefore, the cable 10 is approved for use in telecommunications and electronics applications that require category 5 cable.
Air is provided in the cable 10 in the core 18 and gaps 24-27 and 38, to achieve specific electrical characteristics. The geometric configuration and dielectric constants for the materials used in the cable 10, along with the dielectric constant of air in the core 18 and in air gaps 24-27 and 38 interact to achieve a desired characteristic impedance and to minimize cross talk between signals transmitted over the twisted pairs 14-17, and interact to minimize attenuation and skew. Therefore, the inclusion of air in the cable 10 is necessary and desirable for category 5 cable. By way of example, the cable 10 exhibits standard electrical characteristics.
The cable 10 is able to meet the requirements of the TIA/EIA-568-A standard for the category 5 cable by including air around the insulated conductors 14-17.
In certain networking applications, data transmission protocols may be used that differ from the category 5 standard. For instance, in certain ethernet networks, data transmission protocols are used that meet a less strict standard, such as the 10Base-T standard. By way of example, the ethernet network used at service stations, such as in the example explained above, may utilize a data transmission protocol that satisfies the 10Base-T standard.
A need remains for an improved network cable that is vapor proof and gas impermeable, while continuing to offer the electrical characteristics needed for high speed data transmissions. It is believed that the preferred embodiments of the present invention, satisfy this need and overcome other disadvantages of conventional cabling which will become more readily apparent from the following discussion.
In accordance with at least one preferred embodiment of the present invention, a quad cable is provided including a jacket and at least one quad of insulated signal conductors encased within the jacket. The insulated signal conductors contact one another and are arranged in a helix configuration defining a hollow core. A vapor proof filler substantially fills the hollow core. The jacket and filler fill the gaps and crevices around each insulated conductor to form a hermetic seal along the length of the insulated signal conductors, thereby preventing vapor migration along a length of the cable. In one embodiment, the jacket includes a gas impermeable outer jacket and an inner jacket, while in another embodiment the jacket includes a single unitary jacket. In both embodiments, the single jacket and inner jacket have a dielectric constant higher than a dielectric constant of the insulation on the insulated signal conductors to afford desirable electrical characteristics. The jacket constitutes a pressure extruded compound substantially filling interstices between the insulated signal conductors. The jacket may also include an outer nylon layer substantially impervious to gas. The vapor proof filler represents a pulled core expanded between the insulated signal conductors to substantially fill the hollow core and interstices between the insulated signal conductors. In accordance with one preferred embodiment, the pulled core is formed of cotton, and in an alternative embodiment, the pulled core is formed of an aramid yarn material.
According to an alternative embodiment of the present invention, a method of manufacturing a quad cable is provided. The manufacturing method includes the steps of arranging a quad of insulated signal conductors in a helix and in contact with one another. As the insulated signal conductors are arranged in a helix, they define a hollow core therebetween. The manufacturing method further includes introducing a vapor proof filler between the insulated signal conductors to substantially fill the hollow core and crevices between the insulated signal conductors, before the helix is finally formed. As the helix is formed, the insulated conductors are compressed around the core filler to form a hermetic seal with the inner periphery of the conductors. The method further includes applying a pressure extrudable compound around the outer periphery of the insulated signal conductors as a single or inner jacket. The introducing and applying steps form a seal between the insulated signal conductors, filler and jacket substantially void of air gaps to prevent vapor migration along the length of the insulated signal conductors.
In at least one alternative embodiment, an inner jacket is pressure extruded over the insulated signal conductors. The inner jacket has a dielectric constant higher than a dielectric constant of the insulation on the insulated signal conductors. The pressure extruding step surrounds the outer perimeter of the signal conductors to substantially fill the interstices between the insulated signal conductors with extruded material. The inner layer may be formed from a polyvinylchloride material. The inner jacket may be encased in a gas impermeable outer layer. The outer layer may be formed of a nylon material.
In one alternative embodiment, during the introducing step, the vapor filler is provided between the quad insulated signal conductors before the signal conductors are arranged in a helix and in contact with one another. The vapor proof filler constitutes a soft compressible core. Once the vapor proof filler is properly located between the quad conductors, the quad conductors are compressed and formed into a helix or vice versa. The compression operation causes the vapor proof filler to expand into the grooves between the conductors.